It was a key factor in her charge to the Cincinnati Open title earlier this month.
Analysis from that tournament showed how Gauff is hitting closer to the line but still maintaining her serve speed – underlining a significant improvement in her accuracy.
Gauff now has full confidence her serve can be too hot for opponents to handle – and it showed immediately against Sonmez.
The opening game saw Gauff thump a 112mph ace down the middle, followed by an unreturned first serve and another ace, on her way to holding serve in less than 90 seconds.
Sonmez struggled to get balls into play whenever Gauff landed a first serve in the first set, with the world number four winning 93% of those points.
Gauff broke again for a 1-0 lead in the second set, but her serve was not as effective as Sonmez levelled at 2-2 and held three more break points for a 4-2 lead.
But a precise ace out wide and a 115mph unreturned first serve enabled Gauff to cling on in a game which you felt she would have lost a year ago.
The gutsy hold laid the platform for Gauff to immediately break for 4-3 before securing victory with another service game which displayed her belief.
Gauff hit a trio of unreturned first serves – all just under the 120mph mark, teeing up three match points as Sonmez buckled on the first after a short rally.
“Psychologically, I’m now going to go for the ace, or I know I can hit the serve and the return is probably not going to come back,” said Gauff, who will play Spain’s Paula Badosa or Australia’s Daria Kastakina in the second round.
“Last year I literally was like I hope I make it in. It’s such a big difference knowing almost every serve I’m trying to basically hit 110, 115, 120mph if I can.”